Compare your bases
Look at the exact area where you would stay and the departure points for your activities. A hotel can be on the right island but inconvenient for the route you intend to take. Map the daily transfers before booking.
Choose one main kind of day
For a scenery-focused itinerary, consider which boat trips fit your schedule. If your priority is slower days and food, leave more time near your accommodation and choose excursions selectively.
Check seasons and access
Tourism Authority of Thailand describes November–April as a pleasant period for the Similan Islands on the Andaman side. That does not establish the conditions at every Phuket or Langkawi site. Check local forecasts and current marine-park opening arrangements for the route you choose.
Do not make an international extension automatic
A route between Malaysia and Thailand adds border arrangements. For a short holiday, compare that extra travel with spending another day in one place. Confirm the legal and practical arrangements with the relevant transport provider.
Make a side-by-side budget
Use matching accommodation, meals and activities. Add travel to the gateway and transfers within the island. Keep optional boat days separate so that you can see whether they are driving the difference.
Sources & further reading
Official sources support the factual background. Comparisons, route outlines and budgeting methods are editorial suggestions. Check changing details before travelling.